The release of toxic trace elements from the combustion of fuels in power plants is an environmental issue of increasing concern. Trace element release mechanisms are known to be complex and may depend on the levels of chlorine and sulfur in the fuel burned. This dependency has been investigated experimentally by injecting first hydrogen chlorine, and then sulfur dioxide, into a suspension-firing reactor burning wood-bark under simulated fluidized bed combustion conditions. Data interpretation has been aided by parallel theoretical studies, using a thermodynamic equilibrium model based on the principle of Gibbs free energy minimization. The influence of chlorine and sulfur was confirmed for some, but not all, of the 10 trace elements studied. Injection of HCl served to increase the emission of Cd, but to reduce emission of Cu, Mn, Zn, and possibly Cr and Ni. Injection of SO2 served to increase emissions of Cd, but reduced emissions of As and Hg. The thermodynamic model proved valuable for interpreting the experimental data. However, evidence suggests that predictions not validated by measurement should be treated with caution, given the complexity of the systems being studied.

A suite of improved technologies is being developed to minimize the environmental impact of biomass/waste fired gasification processes. Downdraft, fixed-bed reactors are particularly favored because of their ability to destroy the majority of tars produced from the fuel volatiles. However, there is some concern about the impact of the low residual tar concentration on the long-term operational reliability. A two-stage laboratory scale fixed-bed reactor has been constructed for studying the release and destruction of tars in downdraft gasifiers. The reactor has been commissioned and its performance demonstrated using several biomass feedstocks. Experiments using the first stage only have shown that as the temperature is raised from 250 to 450 °C, the gas and tar yields increase at the expense of the char residue. Four different biomass/waste materials (eucalyptus wood, sludge, plastic waste, and silver birch wood) showed qualitatively similar behavior. Volatile yields appear to stabilize around 450 °C. With silver birch wood, the tar yield reached 47% of the initial fuel. Preliminary tests using a char bed in the second stage have been completed. The presence of the throat and the second-stage char bed results in a substantial reduction in the quantity of tar leaving the reactor. With a hot empty second stage (at 800 °C), the tar content was reduced to 5.3% (by wt of initial fuel charge) in the exit gas from the reactor. Packing the second stage with char (at 800 °C) further decreased the tar content to less than 0.1%. Gas analyses have been performed, showing that some of the initial tar is broken down mainly to CO and CH4 in the second stage of the reactor. Further work is in progress to study the impact of the operating conditions in the second stage on the residual tar concentrations and gas analysis.

The study has involved the investigation of the influence of pyrolysis heat treatment temperature (HTT) and heating rate on the reactivity and the release of char-N during temperature-programmed combustion (TPC) of a set of wire mesh reactor chars in a thermogravimetric analyzer−mass spectrometer system. The gas evolution profiles are bimodal and this indicates the presence of species of different reactivity. It was found that increasing pyrolysis temperature and heating rate both produced significant variation in the reactivity of the resultant chars with the former being more influential. This is apparent from the shift of peak positions and the change in the relative intensity of the low- and high-temperature peaks of CO, CO2, and NO evolution profiles. The differences in the CO2 and NO evolution profiles observed between the Gedling entrained flow reactor (EFR) and wire mesh reactor (WMR) chars of similar heat treatment temperatures may be explained by the different extents of pyrolysis experienced by the chars. However, chars produced in the WMR with pyrolysis temperatures up to 1200 °C show little variation in the char-N conversion to NO. This is believed to be due to the highly reactive nature of the chars which give rise to a high extent of reduction of the primary product NO formed during gasification leading to a low NO/char-N ratio. Heat treatment of the chars at lower heating rates and longer soak times to temperatures in the range 1100−1400 °C lead to reduced char reactivity and higher NO/char-N ratios under temperature-programmed combustion conditions. The results are consistent with the reduction of the primary oxidation product NO on the surface and in the pores of the char.

Pyrolysis and gasification data from two different laboratory reactors have been correlated with results predicted from FT-IR spectra of a set of 26 coals by a statistical data analysis package. Despite the diversity in geological origins of the samples, excellent agreement was obtained between predicted weight loss values and pyrolysis total volatile yields determined in a wire-mesh reactor. Agreement was poor between predicted values and data from a fixed-bed reactor, where the geometry of the reactor appears to have behaved as an interfering variable. The design of the wire-mesh reactor is intended to minimize the effect of reactor geometry on coal pyrolysis yields. This interpretation suggests that the statistical procedure used in this work is capable of leading to predictions of coal pyrolysis yields that may be perceived as physically meaningful. The level of agreement suggests that the initial structures of coals (as reflected in their infrared spectra) are related to their pyrolytic behavior. The method used in this work seems appropriate for estimating volatile matter yields of “unknown” power-station coals under pf-combustion conditions, if a complete set of 1500 °C pyrolysis data are carried out on the “calibration” set in a wire-mesh reactor. However, predictions for conversions in CO2-gasification experiments were poor. FT-IR spectra of coals as the starting point does not seem to be a viable route for reliably predicting CO2-gasification reactivities. Experimentally, the major part of the actual gasification process appears to take place between the reactive gas and the post-pyrolysis char, which has very different properties.